(Reuters) - The U.S. warning to citizens of an increased risk of terrorist attacks in Europe also sent a clear message to militants: "We're on to you."

The repercussions could cut both ways, U.S. officials acknowledged on Monday.

"There is a significant impact on counterterrorism work when the enemy realizes you are on to their game," one U.S. official said, speaking on condition of anonymity.

Known suspects could stop communicating, go underground, or try to weed out spies. A publicly acknowledged threat can also add to confusion for Western intelligence officials, deluging them with questionable tips, misinformation and false alarms.

But the alert could also disrupt plans and prompt militants to re-think an attack.

That makes decisions like Sunday's State Department broadcast of a threat to the general public more complicated.

The trigger for the travel alert was intelligence about a plot against European targets originating in Pakistan's tribal areas. Senior al Qaeda leaders, including Osama bin Laden, are likely involved, U.S. officials say.

The advisory itself was extraordinarily vague, blanketing all of Europe under a generic warning that essentially told U.S. citizens to be on alert. It said public transportation systems and other tourism-related facilities could be targets.

It did not warn citizens against going to Europe, but instead to take precautions if they did.

"As the general nature of the alert suggests, the precise timing and target of potential terrorist operations aren't known," a different U.S. official said.

Paul Pillar, a former CIA counterterrorism official now with Georgetown University, said that so little was revealed in the advisory that fallout was unlikely.

"When what is made public is as general and non-specific as is the case with this advisory about Europe, it's hard to imagine that any specific sources and methods are jeopardized," Pillar said.

At the same time, he added that would-be attackers read newspapers too and that media reports offering more details could tip off militants as well.

"There are opportunities as well as costs," Pillar said.

(Editing by Doina Chiacu)

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Okay, my intelligence career was entirely at the operational level. I was a worker-bee and I enjoyed working with the aircrews and doing the collection, analysis, reporting part of the intelligence cycle and then using the products to provide aircrew training and combat mission briefings. I had once had a dream of being a pilot, so working with the pilots was almost as good. By the time I retired, I was an intelligence "manager" at wing level, a Lieutenant Colonel, but I was still working as a technician, doing the work that I loved. I never did work at any high staff jobs where I had responsibility to set policy or weigh the benefits of disclosing classified information to the public to save lives.

With that perspective, that I am NOT an expert on what I am talking about, I would like to offer my opinion:

The warning that was given to the public was so vague that it was useless. It stirred up fear and probably cancelled some plans for business or vacation travel to Europe.

It gave no hint of how imminent the threat was. All I gathered from the announcements was that it probably involved "Mumbai-style" commando attacks on (maybe) five airports in Germany, France and England.

What was the traveler supposed to do in reaction to that warning?

What did the jihadis learn from the warning? -- Just the fact that we knew that airports in those countries were going to be attacked by commandos told them that we had a source inside their organization.

Is our source safely out and we did this just to screw with their heads and make them carry out a fruitless witch hunt inside their ranks? Or did we take the chance on burning a source because we thought civilian travelers were in imminent danger?

Was the warning meant to throw the jihadis off balance just when they were getting ready to strike? -- Picture a burglar about to climb in a window, shocked when floodlights snap on and alarms start howling.

Bottom line - it was a vague warning so it didn't do the public much good. It may or may not have burned a source. It probably shook up the bad guys. - All the drone missions last month that have been getting 3 -4 jihadis per day in their training area on the Af-Pak border have probably shaken them up a little too.

By the way, one thing to know about intelligence people: when they smile at you and evade the question, change the subject, repeat what you already know, flat out refuse to tell you the answer. --

It's either because: 1. You don't have "a need to know". or 2. They don't know the answer either.